In recent years, dimensional accuracy for photomasks has been rapidly becoming stricter; for example, in-plane dimensional uniformity for photomasks is at most 10 nm. In the steps of manufacturing a photomask, there are many items for determining whether the photomask is acceptable or defective. In the conventional art, if any of the items fails to meet a relevant specification, the photomask is determined to be defective. Thus, the current demanding dimensional accuracy for photomasks inevitably contributes to reducing the yield of photomasks.
Such judgment specifications for photomasks as described above are conventionally determined such that exposure latitude can be achieved even if all the items used for the judgment barely fall within the ranges of specification values. However, for actually created photomasks, it is very rare that all the items barely fall within the ranges of the specification values. Even if any of the items deviates from the range of the specification value, the other items often fall within the ranges of the specification values so as to have the exposure latitude. Thus, the exposure latitude can be achieved even by some of the photomasks for which any of the items deviates from the range of the specification value and which are then determined to be defective and disposed of. That is, even if any of the items for a photomask deviates from the specification values, the photomask may generally achieve the exposure latitude if the other items fall within the ranges of the specification values so as to have the exposure latitude.
Thus, several methods have been proposed in order to avoid above the mask acceptability determination, in which even the mask having a potential to present the exposure latitude is determined defective.